A summer bedroom temperature above 24°C may increase the strain on the heart during sleep, according to a new study.
Above 24°C, Heart Strain Increases
According to a paper published Wednesday in the international journal BMC Medicine, Australian researchers examined the relationship between summer bedroom temperature and cardiac function in 47 adults aged 65 and older (average age 72) living in Queensland, from December 2024 through March of the following year.
Using wearable devices to measure heart rate and heart rate variability (HRV) during sleep, and temperature sensors installed in bedrooms, the researchers analyzed a total of 14,179 hours of sleep data. They found a tendency for cardiac recovery to decline once the temperature exceeded 24°C.
Specifically, when bedroom temperature was between 24 and 26°C, the risk of reduced heart rate variability rose by about 40%. Between 26 and 28°C, it doubled, and above 28°C it surged as much as threefold.
When a person falls asleep, their core body temperature drops as they enter deep sleep. If the bedroom is too hot, this process is disrupted, forcing the heart to pump more blood to lower body temperature and preventing proper rest through the night, the researchers explained.
However, as this is an observational study, it is difficult to conclude that temperature is a direct cause. The study also had limitations, including that participants' use of fans or air conditioners was not confirmed and that the sample of 47 people was small.
But Too Cold Is Also Dangerous
Conversely, running air conditioning too strongly also burdens the heart. According to statistics from the Health Insurance Review and Assessment Service, from December 2020 through August 2025, acute myocardial infarction patients occurred more frequently in summer than in winter.
The five-year combined patient count was 502,086 in summer, about 13,500 more than the 488,506 in winter. Approximately 80% of all patients were male, with those in their 60s making up the largest share.
Acute myocardial infarction refers to a serious emergency condition in which a coronary artery supplying blood to the heart is suddenly blocked by a blood clot, causing heart muscle to die.
When someone moves suddenly from sweltering heat into a strongly air-conditioned indoor space, blood vessels that had widened to release heat narrow instantly, sharply increasing the strain on the heart. During this process, if arteriosclerotic plaque built up on vessel walls ruptures, it can lead to acute myocardial infarction, prompting warnings that caution is needed.
Ultimately, the conclusion converges on finding an appropriate temperature that is neither too hot nor too cold as the key to protecting heart health in summer. Keeping the difference between indoor and outdoor temperatures at around 5°C and keeping a thin outer layer on hand to prevent sudden changes in body temperature are cited as ways to do so.
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